How to move along depth axis on a mac?
You track forward/backward the same way that you scroll a web page. Often, a trackpad does this this with a two-finger drag up and down.

Note, that even when this works correctly, it will still not allow you to view the back side of the cube. By default, it will track toward an invisible pivot point i the center of the view. As it approaches that point, the track will become slower and slower, so that you never go past that point.

If you want to view the opposite side of you cube, you will have to rotate. But there is a setting that makes this a little easier. In User Preferences in the Interface tab, turn on "Rotate Around Selection."



This will force the view to orbit whatever object you have selected, instead of orbiting the center of the view. This will make it MUCH easier to just rotate around to the other side of an object. Check out the other options there, as many of them are very useful.